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Full text of "The Cambridge Natural History"

x                           DEFINITION OF GENUS MQ17L7S                     2 39
we are to continue the plan of defining the various families of
Mammalia.
The gerras Equus * contains not only the Horse, but the Asses
and Zebras. The genus is to be distinguished as regards external
characters by the following features: — The body is thickly
clothed with hair; there is a more or less bushy tail and mane;
the colours are apt to be disposed in stripes of black or blackish
upon a yellowish brown ground; this is of course best seen in
the Zebras, but the wild Asses also have some traces of it, if only
in the single cross-bar of the African Wild Ass, and it is eTen
" reversionary" in the domestic Horse at times. There are no
horns upon the forehead or elsewhere; the fore-limbs or both
pairs have a callous pad upon the inside, which is possibly to be
looked upon as an aborted gland, possibly originally of use as
secreting some odorous substance calculated to enable strayed
members of the herd to regain their companions. The terminal
phalanx of each of the (functionally) single digits is enclosed in
a large horny hoof.
The main internal features of structure which divide this
genus of Perissodactyles from the Rhinoceros or the Tapir, or
from both, are: the existence of strong incisors, three on each
side of each jaw; there are canines, but these are small and do
not always persist in the full-grown mare. They are popularly
known as " tusks " or '* tushes," The first of the four premolars
(the ** wolf tooth ") is smaH and quite rudimentary ; it is often
absent. As there are three molars, the present genus has the
" typical'* number of the Eutherian dentition, i.e. forty-four. In
the skull the orbit is—as it is not in Tapirs and Rhinoceroses—
completely encircled by bone. There is but one functional finger
and toe on each hand (Fig. 121 G) and foot; the second and third
digits are represented by mere splints, one of which may as an
abnormality be enlarged, and reach nearly as far as the well-
developed digit. There are even, occasionally traces of digit
number two.
The Horse, J& ca~bcdlu$9 is to be distinguished from its con-
geners by the small callosities on the hind-limbs which it pos-
sesses in addition to the larger ones on the fore-limbs. The
hairy covering of the tail is more abundant, as is also the mane.
The head too is proportionately smaller, and the general contour
1 Sir W. H. Flower, The Hone, London, 1800.